DETRITAL ZIRCON U-PB GEOCHRONOLOGY PROVIDES CONSTRAINTS ON THE DEPOSITIONAL AGE OF SUR SERIES METASEDIMENTS WITHIN THE SALINIAN TERRANE, CALIFORNIA

Detrital zircon U-Pb geochronology is used to help constrain the depositional age of Sur Series metasedimentary rocks of the Salinian Terrane, CA. The Sur Series metasediments are traditionally believed to be part of the southwestern US Cordilleran miogeocline and potentially the associated Mesozoic cover strata; however, there have been few studies attempting to tightly constrain the depositional ages of these metasedimentary units.

Analysis of detrital zircon U-Pb ages from Sur Series metasedimentary rocks collected from ~300 km of central coastal California suggest a high degree of variability in depositional age. Maximum depositional ages, based on the youngest well-defined group of detrital zircon U-Pb ages, potentially range from Proterozoic to Cretaceous. The Cretaceous zircon U-Pb ages are an interesting age group and three potential sources are considered: 1) Previous researchers have identified dikelets in a number of Sur Series metasedimentary rocks, which could serve as potential contamination, 2) the Cretaceous ages are of metamorphic origin grown during burial of the Sur Series metasediments, and 3) the Cretaceous ages are detrital in origin, constraining the maximum depositional ages.

Although cathodoluminescent images and U/Th provide insight on determining between metamorphic ages grown in situ, igneous ages derived from dikelets, and detrital zircon, further investigation is required to confidently identify the source of the zircon. High U/Th in zircon with Late Cretaceous U-Pb ages suggests that at least some of the samples contain zircon that was grown in situ. A sample of interest, collected from schist exposed along the northwest corner of Point Reyes National Seashore, contains zircon grains with late Early Cretaceous U-Pb ages that have zoning and U/Th indicative of an igneous origin. However, the zircons are older than the proposed Late Cretaceous ages of nearby host plutons and appear to be rounded, suggestive of detrital transport. If the Cretaceous U-Pb ages are detrital, this suggests that some Sur Series strata may have depositional ages considerably younger than previously proposed. We intend to constrain the maximum depositional ages of Sur Series metasediments within the Salinian Terrane and to clarify potential protoliths and source regions for these units.